There are a number of implantable medical devices (IMDs) that sense various physiological parameters and/or provide a variety of therapies. For example, implantable pulse generators (IPG) typically include one or more medical electrical leads that are in contact with cardiac tissue to sense electrical depolarization and provide pacing therapy stimuli. Implantable cardioverter/defibrillators (ICD) also typically include one or more medical electrical leads and provide a larger therapy stimulus for cardioversion or to defibrillate the heart. Often, IMDs include both pacing therapy and cardioversion/defibrillation therapy capabilities.
A housing containing the pulse generator, battery, capacitors, processor, memory, circuitry, etc. is implanted subcutaneously. One or more of the leads are delivered transvenously such that electrodes forming a portion of the lead are disposed within or contacting an outer portion of the heart. The housing, or “can”, may also include one or more electrodes that are selectively used in combination with the various lead electrodes.
In general, the leads sense electrical activity of the heart, typically represented as an electrogram (EGM), which is indicative of the cardiac depolarization waveform and indicates the timing of the various components of the cardiac cycle. This data indicates whether and when intrinsic events occur, their duration and morphology. The timing of certain events (or their failure to occur when expected) is used to trigger various device actions. For example, sensing an atrial depolarization may begin a timer (an escape interval) that leads to a ventricular pacing pulse therapy upon expiration. In this manner, the ventricular pacing therapy is coordinated with respect to the atrial event.
The heart includes four chambers; specifically a right and a left atrium and a right and left ventricle. Leads are commonly and routinely placed into the right atrium as well as the right ventricle. For left sided applications, the lead is typical guided through the coronary sinus and into a cardiac vein. One or more electrodes are then positioned (within the vein) to contact an outer wall of the left atrium and/or left ventricle. Other methods include direct access to the interior of the left atrium and left ventricle through the septal wall.
The therapy delivery circuit in a typical IMD uses at least two capacitors to deliver the therapy stimulus. The sequence is started by charging a capacitor (known as the hold capacitor) to the desired therapy stimulus voltage. Another capacitor (known as the tip capacitor) is typically connected to the tip electrode of the medical lead implanted in the heart. The medical lead has another electrode known as a ring electrode allowing current from the tip electrode to pass through the heart and return to the IMD. During the first phase of a pace therapy stimulus, the previously charged hold capacitor is connected in series with the tip capacitor and the ring electrode. The uncharged tip capacitor begins charging from the hold capacitor. This current passes through the heart (tip-ring electrodes) and provides the therapy stimulus. The second phase connects the tip capacitor to ground allowing the capacitor to discharge. Again, this current passes through the heart and creates the opposite polarity second phase.
In a typical device, this delivery circuit is replicated for each of the leads as necessary with each hold and tip capacitor pair being dedicated to a particular electrode. A need therefore exists for a reduction in the number of components employed by the IMD.